The complex matrix of coral branches creates turbulence in the water, increasing the fall-out of nutritious particles and larvae within the reef. This is the key to why coral reef habitats are amongst the richest in species diversity and abundance. Most of the species found in coral habitats reflects the local fauna and can be found on other substrates. Of the local species pool four groups were found in much larger numbers close to corals; i.e. sponges (Porifera), crustaceans, bristleworms (Polychaeta, Annelida), and 'moss animals' (Bryozoa). But some organisms are extra tightly linked to the coral habitat and is found almost solely in this type of environment. The true 'coral lovers' are presented below.
The polychaete Eunice norvegica (below to the left) can become 25-30 cm long and builds parchment like tubes along the coral branches. The corals then calcify the tubes, making both the coral branches and the tube stronger. Despite the fierce looking fangs (see the image further down on the page) it is supposed to eat foraminiferans (unicellular protozoans). The odd looking hemichordate Rhabdopleura normani is another organism found solely in coral habitats (images to the right below). The larger image shows the colony with its blood red stolon and perpendicular transparent annulated tubes. In these tubes the individual animal resides, and one of them is seen out of its tube on the bottom right image. The brachiopod Macandrevia cranium (bottom, middle) was a regular catch in samples taken in the Koster Fjord up until the 1970's. But with the decline of coral cover it dissapeared, and is now only found at Saekken. Below you also find a squat lobster (Munidopsis serricornis) that is common in the reefs. In tropical reefs they have found that corals benefit from symbiotic relationships with trapezoid crabs. The crabs act as housekeepers and clean the corals from particles too big for them to shed off, and the crabs benefit by getting a safe foraging area between branches. Studies shows that transplanted corals with crabs have better survival than corals without. Munidopsis could be the housekeepers of Lophelia reefs. This type of win-win relationship amongst animals is called mutualistic symbiosis.